Understanding the non-radiative decay mechanism of electron excited states in semiconductor nanocrystals (NCs) is crucial for efficient NC technology. When photo-excitation generates energetic electron-hole pairs, it is necessary to investigate how carriers are converted to band-edge excitons by "thermal exciton cooling". Efficient cooling is achieved by phonon interactions in conventional semiconductor bulk, but the electron-phonon coupling in semiconductor nanocrystals is drastically altered under confinement, and thus the cooling timescales and mechanisms need to be revisited. A team lead by Dr. Dipti Jasrasaria and Prof. Eran Rabani from Department of Chemistry, University of California, USA, developed an atomistic theory to describe hot exciton cooling in II-VI NCs of experimentally relevant sizes. Their framework describes phonon-mediated transitions between excitonic states, which inherently include electron-hole correlations. Furthermore, the authors accurately describe the exciton-phonon couplings (EXPC) and include multiphonon-mediated excitonic transitions. The authors use a master equation approach, which assumes weak EXPC, to propagate exciton population dynamics. The timescales and exciton decay mechanism emerge naturally in the approach. The authors find that cooling occurs on timescales of tens of femtoseconds in wurtzite CdSe NCs, in agreement with measurements, and occurs an order of magnitude slower in wurtzite CdSe-CdS core-shell NCs due to the weaker EXPC in these systems. The authors show that this ultrafast timescale is governed by both electron-hole correlations and multiphonon emission processes, which are made efficient by the quasi-continuous manifold of phonon states in NCs. The results are consistent with the picture emerging from the Auger-assisted relaxation mechanism, but, in addition, the authors attribute the lack of a phonon bottleneck to the important role of multiphonon emission processes.

从声子瓶颈到Auger机制：半导体纳米晶体能量转移的动力学解析